In a wide range of applications, gaseous substances are analyzed and continuously monitored, for example in environmental analysis, in the control of chemical processes, and for detecting chemical warfare agents or explosives in the civil and military fields.
Ion mobility spectrometry (IMS) is a method introduced from the 1970s onward for the highly sensitive detection of substances at low concentrations in air or other gases. An ion mobility spectrometer is operated at ambient pressure and is distinguished by its comparatively compact design, which can be realized with a technically simple set-up, making ion mobility spectrometers particularly suitable as portable and mobile gas monitors and warning devices.
Ion mobility spectrometry has been elucidated in U.S. Pat. No. 3,699,333 (Cohen et al.) and U.S. Pat. No. 4,777,363 (Eiceman), for example.
The measuring tube of the most commonly used time-of-flight type of ion mobility spectrometer comprises a reaction chamber, in which the substances to be analyzed are partially ionized, and a drift chamber in which the ions generated are separated according to their mobility in a drift gas. The two chambers are separated by an electric gating grid. In the reaction chamber, radioactive materials such as 63Nickel are usually used for the primary ionization of gas molecules, but other ionization methods are also possible, for example a corona discharge. The ionization of the substances to be analyzed typically takes place only in secondary reactions. The ions generated in the reaction chamber are admitted into the drift chamber by opening the electric gating grid for a short time. There the ions move through the drift gas in an axial electric field to the other end of the drift chamber, where they are measured as an ion current at a collecting electrode. The ions are temporally separated in the drift chamber because their speed in the drift gas is substance-specific. The time delays of the ion current measured at the collecting electrode with reference to the opening of the electric gating grid are used to determine the substance-specific drift times as a measure of ion mobility. When parameters such as the gas temperature and gas pressure are taken into account, the drift times are characteristic of the respective substances. In the measuring tube, the gas is guided in such a way that the gaseous substances to be analyzed preferably only reach the reaction chamber, and the cleaned drift gas flows through the drift chamber in the opposite direction to the ion drift.
In other types of ion mobility spectrometer, for example the filter types (FAIMS: “Field Asymmetric Ion Mobility Spectrometer”) or the multi-electrode types (aspiration type), the ions move in electric fields aligned radially or transversely to the longitudinal axis of the drift chamber. A drift gas which flows through the drift chamber at right angles to the electric fields transports the ions longitudinally through the drift chamber.
In order to achieve reproducible results, particularly with an ion mobility spectrometer of the time-of-flight type, the drift gas, in which the ions move, must be continuously cleaned and freed of moisture, which is carried into the ion mobility spectrometer by the sample gas introduced.
An alternative gas exchange by feeding from supply tanks or external gas processing units is primarily suitable for stationary operation. In most mobile ion mobility spectrometers the drift gas is guided in a closed loop circulatory gas system through a filter with the help of a transport device such as a gas pump or a fan. This removes the moisture from the gas in the circulatory gas system and cleans it before it is conveyed back again to the measuring tube of the ion mobility spectrometer. Ion mobility spectrometers of the filter and multi-electrode type typically have no circulatory gas system, and until now have been mainly flushed directly with the sample gas introduced. However, they often exhibit a saturation effect even at substance concentrations relevant to use in the field, which limits their practical range of application.
With an ion mobility spectrometer with a circulatory gas system, the defined introduction of sample gas for analysis out of ambient gas that is not at a higher pressure is often a task which is beset with problems. In the case of a mobile ion mobility spectrometer, the sample gas to be analyzed is preferably introduced into the circulatory gas system either upstream of, or in the area of, the measuring tube, where it mixes with the gas of the circulatory gas system. The following technical variants have been elucidated:                a) permeation of the gaseous substances to be detected through a membrane that is semipermeable to these substances and is flushed from outside with sample gas (U.S. Pat. No. 4,311,669 by Spangler et al.),        b) introduction of gas loops or vessels charged with sample gas into the circulatory gas system with the help of valve actuations (DE Patent 195 02 674 by Leonhardt et al.),        c) aspiration of the sample gas with the help of a vacuum pump connected to a valve actuation (U.S. Pat. No. 5,811,059 by Genevese et al.),        d) periodic aspiration of sample gas through a narrow aperture with the help of low-pressure pulses generated by a pump membrane (WO Patent 93/06476 by Bradshaw) and        e) feeding in sample gas by means of a perforated disk made of gas-impermeable material, arranged between the ambient gas and the measuring tube of the ion mobility spectrometer, the disk being actively set in oscillation to generate periodic pressure differences (EP Patent 0 774 663 by Grossniklaus et al.).        
Experience has shown that using a semipermeable membrane, as done by Spangler et. al, for example, requires electrical heating in order to minimize delay and accumulation effects in the membrane material. The energy required for heating reduces the operating time, especially in the case of battery-operated mobile gas monitors, without completely eliminating the accumulation effects and the delayed reaction to variations in the external concentration. Moreover, the manufacture and assembly of the thin semipermeable membranes is complex and expensive. Furthermore, membranes of this type are mechanically sensitive because of the small layer thicknesses which are required.
The technical variants b) to e) are direct inlet systems which introduce the sample gas to be analyzed directly into the circulatory gas system, and thus avoid significant disadvantages of the membrane systems. One disadvantage of the direct inlet systems used until now, however, is that they additionally require special transport devices or valves plus the associated actuating components (such as motors, solenoids or piezoelectric actuators) to aspirate sample gas either continuously or in pulses. This makes manufacturing complex and costly, and also involves additional energy consumption, which in turn reduces the operating time of a set of batteries in battery-operated instruments, or forces one to use larger and heavier batteries, making the instruments less compact. In variant a), in addition to the transport device in the circulatory gas system, a further transport device is used to flush the semipermeable membrane.
In direct inlet systems without a membrane, the accumulation of low-volatility substances in the inlet region and on the inner wall of the measuring tube of the ion mobility spectrometer still occurs, to a lesser degree, as a result of condensation, adsorption and solubility effects. Consequently, these systems are also saturated for a certain time after a large amount of substance has been admitted, and are thus no longer ready to measure for a certain time. This is particularly problematic in the set-up of valves described in DE Patent 195 02 674 (Leonhardt et al.) since, in this case, the sample gas to be analyzed flows through further valves upstream of the gas analyzer. The method usually used in stationary instruments, and especially in membrane systems, to minimize these accumulation effects is to heat the inlet region, but this is only possible to a limited extent in mobile instruments because of the energy required. To accelerate the desorption of the accumulated substances, the only other possibility remaining is therefore the specific flushing of the measuring tube and the inlet region with a cleaned gas stream, which is directed toward the introduced sample gas in the inlet region. Gas aspirated from outside, which is cleaned in a special back flush filter, is used for the so-called back flushing. The systems used up to now require further components (e.g. valves and pumps), however, which again complicate manufacture and create additional energy consumption.
A further disadvantage of the direct inlet systems from the WO patent 93/06476 (Bradshaw) and the EP patent (Grossniklaus et al.) is the fact that the gas streams directed inward and outward, which are caused by the periodic high- and low-pressure phases of the pump membrane or oscillating perforated disk, only transport very small amounts of gas. The volume of the dosing channel is kept as small as possible here so that, during a low-pressure phase, sufficient sample gas reaches the inside of the measuring tube of the ion mobility spectrometer.